An English study of more than 11,000 children has turned up an association between early antibiotic use (that is, antibiotics given to babies less than 6 months of age) and later obesity. Interestingly, the study did not find that antibiotics given to children between the ages of 6 and 14 months increased the risk of obesity, and the effect of antibiotics on children aged 15-23 months was inconsistent.

Why would the antibiotic-obesity association be found primarily in younger babies? The authors speculate that an altered gut microbiota may be the culprit.

The germs that make up the gut microbiota (GM) are acquired at birth and shortly afterwards. By a few months of age the “core” GM is more or less set for life. An altered GM has long been associated with obesity in older children and adults (see more extended discussions in my posts here and here)–it would make sense that antibiotics given in this sensitive period of GM development would have greater impact than later on, when the GM is more stable.

The added risk of obesity from early antibiotic administration is small for any individual baby, the study’s authors stress, but even small increases spread over an entire population can have significant public health implications.